Analysis of melatonin in blood has been controversal; many assays exist and there is little quantitative agreement among laboratories. In our laboratory, we found that our quantitative determination of melatonin in plasma was not consistent with our previous observations. After extensive re-assessment of the assay, we found that alteration in pH of one wash solution altered, by 2 orders of magnitude, the quantitative estimation of the concentration of plasma melatonin.
